Method for setting up a data transmission link between xDSL transceivers

ABSTRACT

The invention relates to a method for setting up a data transmission link between xDSL transceivers, comprising the following steps: sending of an echo adjustment signal (W ECL ) by a first xDSL transceiver ( 1 - 1 ) to a second xDSL transceiver ( 1 - 2 ) until an echo cancelation circuit ( 21 - 1 ) of the first xDSL transceiver ( 1 - 1 ) is set, the second xDSL transceiver ( 1 - 2 ) synchronizing to the echo adjustment signal (W ECL ) and the second xDSL transceiver ( 1 - 2 ) not sending; sending of a first synchronization signal (W SL ) via the first xDSL transceiver ( 1 - 1 ) to the second xDSL transceiver ( 1 - 2 ) for synchronizing the second xDSL transceiver ( 1 - 2 ), the first synchronization signal (W SL ) being different from the echo adjustment signal (W ECL ); sending of a second synchronization signal (W SN ) by the second xDSL transceiver ( 1 - 2 ) to the first xDSL transceiver ( 1 - 1 ) for synchronizing the first xDSL transceiver ( 1 - 1 ) when the second xDSL transceiver ( 1 - 2 ) is completely synchronized by the first synchronization signal (W SL ), the second synchronization signal (W SN ) being sent by the second xDSL transceiver ( 1 - 2 ) to the first xDSL transceiver ( 1 - 1 ) until an echo cancelation circuit ( 21 - 2 ) of the second xDSL transceiver ( 1 - 2 ) is set, sending of a first indicating signal (W OKN ) by the second xDSL transceiver ( 1 - 2 ) to the first xDSL transceiver ( 1 - 1 ), which indicates to the first xDSL transceiver ( 1 - 1 ) that the echo cancelation circuit ( 21 - 2 ) of the second xDSL transceiver ( 1 - 2 ) is set and the second xDSL transceiver ( 1 - 2 ) is synchronized; and sending of a second indicating signal (W OKL ) by the first xDSL transceiver ( 1 - 1 ) to the second xDSL transceiver ( 1 - 2 ) which indicates to the second xDSL transceiver ( 1 - 2 ) that the first xDSL transceiver ( 1 - 1 ) is synchronized when the first xDSL transceiver ( 1 - 1 ) has received the first indicating signal (W OKN ).

[0001] The invention relates to a method for setting up a datatransmission link between xDSL transceivers with a minimum warm startperiod.

[0002]FIG. 1 shows a data transmission system according to the priorart. Between a transceiver at the switch end LTU (Line Terminating Unit)and a transceiver at the subscriber end NTU (Network Terminating Unit),the signaling data are first transmitted via a data transmission linefor setting up a data transmission link for user data. The transceiverat the subscriber end NTU is a connecting socket which is installed incustomer or subscriber premises by the network operator. The transceiverat the subscriber end NTU is connected to a data terminal, for example acomputer. The transceiver at the switch end LTU is connected to anetwork, for example the telephone network. As a rule, the datatransmission line is a copper telephone pair.

[0003] To set up a data transmission link, the two transceivers must bebrought from a standby mode into an activated operating mode for thedata transmission. In xDSL data transmission methods, the twisted analogtelephone pairs are used as wide-band connections in the local loop. Themost well-known xDSL data transmission methods are ADSL (AsymmetricDSL), VDSL (Very-High-Data-Rate DSL), SDSL (Symmetrical Single-PairDSL).

[0004] After the completed cold start and subsequent deactivation, thetwo transceivers are initially in a power-saving or standby mode. In thepower-saving mode, the transceivers consume minimum power so that theheat generated is minimum and no cooling is necessary.

[0005]FIG. 2 shows a link between two transceivers LTU, NTU according tothe prior art in detail. To set up a data link, the transceivers arereactivated in a warm-start sequence.

[0006]FIG. 3 shows a possible warm-start sequence according to the priorart, which is described in German Patent Application DE 101 39 779.8.

[0007] Initially, one of the two transceivers sends a wake-up signal viathe data transmission line to the remote transceiver in order toactivate the latter. In the sequence shown in FIG. 3, the transceiver atthe subscriber end NTU sends a wake-up signal W_(WUN) to the transceiverat the switch end LTU. After a predetermined interval T_(WS), thetransceiver at the switch end LTU also sends a wake-up signal W_(WUL)back to the modem at the subscriber end NTU via the data transmissionline. With this wake-up signal, the modem at the switch end LTUacknowledges reception of the original wake-up signal W_(WUN).

[0008] After the wake-up sequence, it is determined in a line probingphase whether the line parameters of the data transmission line havechanged since the last activation. For this purpose, the transceiver atthe subscriber end NTU sends a line-probing signal W_(LPN) via thetransmission line to the transceiver at the switch end LTU in theexample shown in FIG. 3. The transceiver at the switch end evaluates thereceived signal. After a predetermined waiting time T_(WS), thetransceiver at the switch end LTU conversely also sends a line-probingsignal W_(LPL) to the transceiver at the subscriber end NTU. The modemat the subscriber end NTU evaluates the received signal.

[0009] After the line parameters have been checked, a complete echosignal elimination may take place in a further phase. Since the lastactivation of the two transceivers, the line parameters of the datatransmission line may have changed. This leads to a residual echo signalof the inherent transmit signal of a transceiver. The residual echosignal may impair the detection of a data signal received from theremote transceiver. The echo signal must, therefore, be canceled by echosignal elimination in such a way that it is below a predeterminedthreshold value. To eliminate the echo signal, the modem at thesubscriber end NTU in the example shown in FIG. 3 sends out a signal forecho cancelation and the remote modem at the switch end LTU does notsend out a signal. The echo cancelation circuit in the modem at thesubscriber end NTU is set with the aid of the W_(ECN) signal. After apredetermined waiting time t_(WS) [sic], the modem at the switch end LTUthen also sends out a signal W_(ECL) for echo signal cancelation. Duringthis time, the other modem NTU does not send out a signal. The echocancelation circuit in the transceiver at the switch end LTU is adjustedto minimize the residual echo signal with the aid of the adjustmentsignal W_(ECL).

[0010] After the echo signal elimination, synchronization is effectedbetween the two transceivers in a synchronization sequence. The modem atthe subscriber end NTU sends out a synchronization signal W_(SN) forsynchronizing the transceiver at the switch end LTU and conversely themodem at the switch end LTU sends out a synchronization signal W_(SL)for synchronizing the modem at the subscriber end NTU. As soon as thetwo transceivers are synchronized, they in each case send an indicatingsignal to the other transceiver which indicates the completedsynchronization. Following this, user data are transmitted between thetwo transceivers.

[0011] For a warm start between two transceivers, it is desirable thatthe period for the warm start t_(warm start) is as short as possible. Inthe warm start sequence according to the prior art, the probing of thedata transmission line, the echo signal elimination and thesynchronization take place after one another or serially. A seriousproblem in this is that a predetermined worst-case time is in each caseprovided for probing the data transmission line and for eliminating theecho signal in the standard protocol for setting up the datatransmission link. For this reason, the period for the warm start isrelatively long in the warm start sequence according to the prior art.

[0012] It is, therefore, the object of the present invention to create amethod for setting up a data transmission link between xDSL transceiversin which the period for setting up the data transmission link isminimum.

[0013] According to the invention, this object is achieved by a methodaccording to the features specified in claim 1 and by an xDSLtransceiver having the features specified in claim 14.

[0014] The invention creates a method for setting up a data transmissionlink between xDSL transceivers comprising the following steps: sendingof an echo adjustment signal (W_(ECL)) by a first xDSL transceiver (1-1)to a second xDSL transceiver (1-2) until an echo cancelation circuit(21-1) of the first xDSL transceiver (1-1) is set, the second xDSLtransceiver (1-2) synchronizing to the echo adjustment signal (W_(ECL))and the second xDSL transceiver (NTU) not sending; sending of a firstsynchronization signal (W_(SL)) via the first xDSL transceiver (1-1) tothe second xDSL transceiver (1-2) for synchronizing the second xDSLtransceiver (1-2), the first synchronization signal (W_(SL)) beingdifferent from the echo adjustment signal (W_(ECL)); sending of a secondsynchronization signal (W_(SN)) by the second xDSL transceiver (1-1) tothe first xDSL transceiver (1-2) for synchronizing the first xDSLtransceiver (1-1) when the second xDSL transceiver (1-2) is completelysynchronized by the first synchronization signal (W_(SL)), the secondsynchronization signal (W_(SN)) being sent by the second xDSLtransceiver (1-2) to the first xDSL transceiver (1-1) until an echocancelation circuit (21-2) of the second xDSL transceiver (1-2) is set,sending of a first indicating signal (W_(OKN)) by the second xDSLtransceiver (1-2) to the first xDSL transceiver (1-1), which indicatesto the first xDSL transceiver (1-2) that the echo cancelation circuit(21-2) of the second xDSL transceiver (1-2) is set and the second xDSLtransceiver (1-2) is synchronized; and sending of a second indicatingsignal (W_(OKL)) by the first xDSL transceiver (1-1) to the second xDSLtransceiver (1-2) which indicates to the second xDSL transceiver (1-2)that the first xDSL transceiver (1-1) is synchronized when the firstxDSL transceiver (1-1) has received the first indicating signal(W_(OKN)).

[0015] An advantage of the method according to the invention consists inthat it exhibits a very low susceptibility to interference because thevarious steps for setting up the data transmission link, namely probingof the data transmission line, echo signal elimination andsynchronization in each case take place sequentially at the twotransceivers and thus interfering mutual influence during theadjustments of the echo cancelation circuits of the equalizers and ofthe synchronization circuits are avoided.

[0016] Advantageous embodiments of the method according to the inventionare specified in the subclaims.

[0017] The invention also creates an xDSL transceiver comprising asignal input for receiving digital user data, a signal generator forgenerating signaling data,

[0018] a controllable multiplexer for switching through the generatedsignaling data or the user data in dependence on a control signal,

[0019] a scrambler for scrambling the data switched through by themultiplexer,

[0020] a mapping unit for mapping the scrambled data,

[0021] a coder for coding the mapped data,

[0022] a D/A converter for converting the coded data into an analogtransmit signal,

[0023] a hybrid circuit for connecting the xDSL transceiver via a datatransmission line to a remote xDSL transceiver,

[0024] an analog/digital converter for converting an analog receivedsignal into a digital received signal,

[0025] a detection circuit for detecting a received wake-up signal,

[0026] a synchronization circuit for synchronizing to the receiveddigital signal,

[0027] an echo cancelation circuit for echo signal cancelation in thereceived digital signal,

[0028] an equalizer for equalizing the received digital signal,

[0029] a descrambler for descrambling the equalized received digitalsignal,

[0030] and with a control unit,

[0031] the control unit, after a received wake-up signal has beendetected by the detection circuit or after a warm start request signalhas been received, controlling the coder in such a manner that an echoadjustment signal W_(ECL) is sent by the xDSL transceiver via the datatransmission line, the control unit, after receiving a first indicatingsignal from the echo cancelation circuit (W_(ECOK)), which indicatesthat the echo cancelation circuit (EC) is set, driving the multiplexerin such a manner that a generated synchronization data sequence isswitched through to the scrambler which is sent as synchronizationsignal (W_(SL)) by the xDSL transceiver via the data transmission linefor synchronizing the remote xDSL transceiver,

[0032] the control unit, after receiving a second indicating signal(W_(SYNCOK)) from the synchronization circuit, which indicates that thesynchronization circuit is synchronized to a received digital signal andafter receiving a third indicating signal (W_(EQOK)) from the equalizer(EQ), which indicates that the equalizer (EQ) is set, driving themultiplexer in such a manner that a generated synchronization datasequence is switched through to the scrambler which is sent asindicating signal (W_(OK)) by the xDSL transceiver via the datatransmission line to the remote xDSL transceiver and indicates to thelatter that the xDSL transceiver is ready for the data transmission ofuser data.

[0033] In the text which follows, preferred embodiments of the methodaccording to the invention for setting up a data transmission link andof the xDSL transceiver according to the invention are described withreference to the attached figures for explaining features which areessential to the invention. In the figures:

[0034]FIG. 1 shows a data transmission link according to the prior art;

[0035]FIG. 2 shows a circuit configuration of conventional transceivers;

[0036]FIG. 3 shows a timing diagram which represents the setting-up of adata transmission link according to the prior art;

[0037]FIG. 4 shows a block diagram of an xDSL transceiver according tothe invention;

[0038]FIG. 5 shows a flow chart for explaining the method according tothe invention for setting up a data transmission link;

[0039]FIG. 6 shows a state diagram for explaining the method accordingto the invention for setting up a data transmission link between twoxDSL transceivers.

[0040]FIG. 4 shows a block diagram of the xDSL transceiver 1 accordingto the invention. The xDSL transceiver 1 has at least one signal input 2for receiving digital user data. The digital user data come either froma data terminal or from a data network. The xDSL transceiver 1 alsocontains a controllable multiplexer 3. The controllable multiplexer 3has a first input 4 which is connected to the signal input 2 for theuser data via a line 5. In addition, the multiplexer 3 has a secondinput 6 which is connected to a signaling data generator 8 via a line 7.The multiplexer 3 is switched by an internal controller 10 of the xDSLtransceiver 1 via a control line 9. The multiplexer 3 has a signaloutput 11 which is connected to a scrambler 13 via a line 12.

[0041] The output of the scrambler 13 is connected via a line 14 to amapping unit 15 for mapping the scrambled digital data. The output ofthe mapping unit 15 is connected via a line 16 to a coder 17 for codingthe mapped data.

[0042] In a preferred embodiment of the xDSL transceiver according tothe invention, the coder 17 is a so-called Tomlinson coder. The codeddata are passed to a digital/analog converter 19 via a line 18 and to anecho cancelation circuit 21 via a line 20. The digital/analog converter19 converts the coded data present into an analog transmit signal whichis applied to a driver circuit 23 of the xDSL transceiver 1 via aninternal line 22. The driver circuit 23 amplifies the analog signalpresent and outputs the amplified signal to the hybrid circuit 25 via aline 24. The hybrid circuit 25 forms the connection between the xDSLtransceiver 1 and the data transmission line 26. The data transmissionline 26 is preferably a copper pair.

[0043] The hybrid circuit 25 is connected via a line 27 to ananalog/digital converter 28 which converts the received analog signalinto received digital data and outputs them to a subtracting unit 30 viaa line 29. The subtracting unit 30 subtracts the output signal of theecho cancelation circuit 21, which is present via a line 31, from thereceived digital data. The subtracting unit 30 is followed by anequalizer 33 via a line 32. The equalizer is used for far-end signalrecovery and equalizes the signal distortions occurring. The outputsignal of the subtracting unit 30 is output to a synchronization circuit35 of the xDSL transceiver 1 via a line 34. The synchronization circuit35 is used for synchronizing the xDSL transceiver 1 to a receivedsignal. The synchronization circuit 35 is followed by a decoding anddescrambler circuit 37 via a line 36. The decoder/descrambler 37 decodesand descrambles the equalized received data and outputs the descrambleddata to a signal output 39 of the xDSL transceiver 1 via a line 38.

[0044] The xDSL transceiver 1 also contains a detection circuit 40 whichis provided for detecting a received wake-up signal. The detectioncircuit 40 outputs a detection signal to the controller 10 via a line 41when a wake-up signal or wake-up tone signal is received via the datatransmission line 26. The controller 10 controls the signaling datagenerator 8 via a control line 42. In addition, the controller 10controls the coder 17 via lines 43. The echo cancelation circuit 21 andthe equalizer circuit 33 in each case indicate to the controller 10, viaindicating lines 44, 45, that they are set in accordance with their owncriteria. The echo cancelation circuit 21 indicates via the line 44 whenit is converged, i.e. when the residual echo signal is below a thresholdvalue.

[0045] The equalizer circuit 33 indicates to the controller 10 via theline 45 when the far-end signal is sufficiently equalized and theequalizer 33 is thus set.

[0046] In addition, the synchronization circuit 35 indicates to thecontroller 10 via a line 46 when adequate synchronization to thereceived data signal has taken place. In addition, the controller 10 canreceive a warm start request signal via a line 47.

[0047]FIGS. 5, 6 show the sequence of the method according to theinvention for setting up a data transmission link between two xDSLtransceivers 1 according to the invention.

[0048] Initially, one of the two xDSL transceiver 1 sends a wake-upsignal W_(WUN) to wake up the other transceiver which acknowledges thereception of the wake-up signal by sending out a wake-up signal W_(WUL).

[0049] After a certain waiting time T_(WS), the synchronization sequenceaccording to the invention begins with the setting-up of the datatransmission link. In a step S1, a first xDSL transceiver 1-1 sends anecho adjustment signal W_(ECL) to a second xDSL transceiver 1-2 untilthe echo cancelation circuit 21-1 of the first xDSL transceiver 1-1 isset, during which process the second xDSL transceiver 1-2 alreadysynchronizes itself to this echo adjustment signal W_(ECL). During thisperiod, the second xDSL transceiver 1-2 is not sending a signal.

[0050] Once the echo cancelation circuit 21-1 is set, the xDSLtransceiver 1-1 sends a synchronization signal W_(SL), which differsfrom the echo adjustment signal W_(ECL) and which indicates to thesecond xDSL transceiver 1-2 that the echo cancelation circuit 21-2 ofthe first xDSL transceiver 1-1 is set. For this purpose, the controller10 drives the signaling data generator 8 via the control line 42 in sucha manner that it applies a different signaling data sequence to theinput 6 of the multiplexer 3 via the line 7. The multiplexer 3 isswitched by the controller via the control line 9 in such a manner thatthe generated signaling data sequence is switched through to the output11 of the multiplexer 11. The user data input 4 of the multiplexer 3 isblocked during this sequence. The signaling data sequence generated isscrambled by the scrambler 13 and then mapped by the mapping unit 15.The mapped signaling data present are coded by the coder 17 andconverted into the first synchronization signal W_(SL) by thedigital/analog converter 19.

[0051] When the second xDSL transceiver 1-2 at the other end of the datatransmission line 26 is completely synchronized by the firstsychronization signal W_(SL) and has recognized the firstsynchronization signal W_(SL) as such by comparing it with thepredetermined data bit sequence, the second xDSL transceiver 1-2 sends asecond synchronization signal W_(SN) to the first xDSL transceiver 1-1for synchronizing the first xDSL transceiver 1-1. During this process,the second synchronization signal W_(SN) is sent by the second xDSLtransceiver 1-2 to the first xDSL transceiver 1-1 until the echocancelation circuit 21-2 of the second xDSL transceiver is set.

[0052] The second xDSL transceiver 1-2 then sends to the first xDSLtransceiver 1-1 a first indicating signal W_(OKN) which indicates to thefirst xDSL transceiver 1-1 that the echo cancelation circuit 21-2 of thesecond xDSL transceiver 1-2 is set and the second xDSL transceiver 1-2is synchronized. Conversely, the first xDSL transceiver 1-1 sends to thesecond xDSL transceiver 1-2 a second indicating signal W_(OKL) whichindicates to the second xDSL transceiver 1-2 that the xDSL transceiver1-1 is synchronized when the first xDSL transceiver 1-1 has received thefirst indicating signal W_(OKN).

[0053] In a preferred embodiment of the method according to theinvention, the transmit signals are Tomlinson-coded by the coder 17. Ifno Tomlinson preceding takes place, a further synchronization sequence 2may be required as shown in FIG. 5. The signals W_(SN2), W_(SL2),W_(OKN2), W_(OKL2) are here identical with the non-Tomlinson-precodedsignals of the synchronization sequence 1 but the signals areTomlinson-precoded in the synchronization sequence 2.

[0054] As can be seen by comparing the timing diagrams of FIG. 3 andFIG. 5, the warm start period t_(warm start) required is much shorter inthe method according to the invention for setting up a data transmissionlink, particularly with Tomlinson coding of the signaling data, than inthe conventional method shown in FIG. 3.

[0055] In the method according to the invention, the echo adjustmentsignal W_(ECL) is structured in such a manner that the remotetransceiver 1 can already synchronize to it. In the method according tothe invention, the echo adjustment signal W_(EC) fulfills a dualfunction. On the one hand, the echo adjustment signal W_(ECL) is usedfor adjusting the local echo cancelation circuit 21 and, on the otherhand, for synchronizing the other xDSL transceiver 1 through itssynchronization circuit 35. If necessary, the remote transceiver 1 isthen completely synchronized by the first synchronization signal W_(SL)sent which differs from the echo adjustment signal W_(ECL). In contrastto the previous method, the time t_(ECL) is not predetermined as aconstant time in the method according to the invention. The echoadjustment signal W_(ECL) is sent out by the xDSL transceiver 1 onlyuntil its echo cancelation circuit 21 is completely set and it indicatesthose to the controller 10 via the indicating line 44. The controller 10then drives the signal data generator 8 via the control line 42 in sucha manner that it generates another signaling data sequence for the firstsynchronization signal W_(SL). In the method according to the invention,the echo signal elimination thus takes place within the synchronizationsequence so that the necessary warm start period t_(warm start) isconsiderably reduced in the method according to the invention.

[0056]FIG. 6 is used for explaining the method according to theinvention and shows a state diagram of the two communicatingtransceivers NTU, LTU. The two transceivers are initially in a standbyor power-saving mode. If the controller 10 of a transceiver receives awarm start request signal via the line 47 or detects a wake-up signalwhich originates from the connected other xDSL transceiver 1 via thedata transmission line 26, the transceiver leaves the standby mode.

[0057] After the exchange of the wake-up signals, the modem at theswitch end LTU sends an echo adjustment signal W_(ECL) in the sequenceshown in FIG. 6, to which the modem at the subscriber end NTU canalready synchronize. The transceiver at the switch end NTU adjusts itsecho cancelation circuit 21 by means of the echo adjustment signalW_(ECL). Once the echo cancelation circuit 21 is set, this is reportedto the controller 10 which drives the signal generator 8 to generateanother signaling data sequence.

[0058] The transceiver 1 at the switch end LTU then sends asynchronization signal W_(SL) via the data transmission line 26. Thesynchronization signal W_(SL) adjusts the equalizer 35 of the othertransceiver 1. After the equalizer 35 is set and the synchronizationsignal W_(SL) is detected, the transceiver 1 at the subscriber end NTUchanges into another operating mode and now sends a synchronizationsignal W_(SN). This signal W_(SN) is received by the transceiver at theswitch end LTU and the transceiver LTU synchronizes to the receivedsynchronization signal W_(SN). During this sequence, the echocancelation circuit EC_(N) of the transceiver at the subscriber end NTUis adjusted.

[0059] After the echo cancelation circuit EC_(N) is set, the transceiverat the subscriber end NTU sends an indicating signal W_(OKN) via thedata transmission line 26. If the equalizer 33 of the modem at theswitch end LTU is set and the LTU transceiver detects the indicatingsignal W_(OKN) of the other transceiver, it also sends an indicatingsignal W_(OKL) to the opposite transceiver. Once the transceiver at thesubscriber end NTU has detected the indicating signal W_(OKL), bothtransceivers are ready for exchanging user data via the datatransmission line 26.

[0060] The method according to the invention is a handshake methodbetween the two xDSL transceivers 1. In this method, no timingconditions are predetermined a priori. The progress of activation ofeach transceiver is additionally controlled by the control signal fromthe remote transceiver in addition to the internal conditions achieved.During this process, the unwanted loss of time due to the exchange ofinformation or control signals is minimized by parallel processing ofthe sequences. Both transceivers can adjust to their own echocancelation circuit 21 without control signals from the opposite end. Atthe same time, the opposite end synchronizes and adapts itself.

[0061] List of Reference Designations

[0062]1 xDSL transceiver

[0063]2 Signal input

[0064]3 Multiplexer

[0065]4 Input

[0066]5 Line

[0067]6 Input

[0068]7 Line

[0069]8 Signaling data generator

[0070]9 Control line

[0071]10 Controller

[0072]11 Output

[0073]12 Line

[0074]13 Scrambler

[0075]14 Line

[0076]15 Mapping unit

[0077]16 Line

[0078]17 Coder

[0079]18 Line

[0080]19 Digital/analog converter

[0081]20 Line

[0082]21 Echo cancelation circuit

[0083]22 Line

[0084]23 Driver

[0085]24 Line

[0086]25 Hybrid circuit

[0087]26 Data transmission line

[0088]27 Line

[0089]28 Analog/digital converter

[0090]29 Line

[0091]30 Subtracting unit

[0092]31 Line

[0093]32 Line

[0094]33 Equalizer

[0095]34 Line

[0096]35 Synchronization circuit

[0097]36 Line

[0098]37 Descrambler

[0099]38 Line

[0100]39 Output

[0101]40 Detection circuit

[0102]41 Indicating line

[0103]42 Control line

[0104]43 Control line

[0105]44 Indicating line

[0106]45 Indicating line

[0107]46 Indicating line

[0108]47 Control line

1. A method for setting up a data transmission link between xDSLtransceivers (1-1, 1-2), comprising the following steps: a) sending ofan echo adjustment signal (W_(ECL)) by a first xDSL transceiver (1-1) toa second xDSL transceiver (1-2) until an echo cancelation circuit (21-1)of the first xDSL transceiver (1-1) is set, the second xDSL transceiver(1-2) synchronizing to the echo adjustment signal (W_(ECL)) and thesecond xDSL transceiver (1-2) not sending; b) sending of a firstsynchronization signal (W_(SL)) via the first xDSL transceiver (1-1) tothe second xDSL transceiver (1-2) for synchronizing the second xDSLtransceiver (1-2), the first synchronization signal (W_(SL)) beingdifferent from the echo adjustment signal (W_(ECL)); c) sending of asecond synchronization signal (W_(SN)) by the second xDSL transceiver(1-2) to the first xDSL transceiver (1-1) for synchronizing the firstxDSL transceiver (1-1) when the second xDSL transceiver (1-2) iscompletely synchronized by the first synchronization signal (W_(SL)) andthe first synchronization signal is detected as such, the secondsynchronization signal (W_(SN)) being sent by the second xDSLtransceiver (1-2) to the first xDSL transceiver (1-1) until an echocancelation circuit (21-2) of the second xDSL transceiver (1-2) is set,d) sending of a first indicating signal (W_(OKN)) by the second xDSLtransceiver (1-2) to the first xDSL transceiver (1-1), which indicatesto the first xDSL transceiver (1-1) that the echo cancelation circuit(21-2) of the second xDSL transceiver (1-2) is set and the second xDSLtransceiver (1-2) is synchronized; e) and sending of a second indicatingsignal (W_(OKL)) by the first xDSL transceiver (1-1) to the second xDSLtransceiver (1-2) which indicates to the second xDSL transceiver (1-2)that the first xDSL transceiver (1-1) is synchronized when the firstxDSL transceiver (1-1) has received the first indicating signal(W_(OKN)).
 2. The method as claimed in claim 1, wherein the echocancelation circuits (21) are adjusted until a residual echo signaldrops below a threshold value.
 3. The method as claimed in claim 1 or 2,wherein the second xDSL transceiver (1-2) does not send a signal as longas the first xDSL transceiver (1-1) is sending the echo adjustmentsignal (W_(ECL)).
 4. The method as claimed in one of the precedingclaims, wherein an equalizer (33-2) of the second xDSL transceiver (1-2)is adjusted by the first synchronization signal (W_(SL)) and anequalizer (33-1) of the first xDSL transceiver (1-1) is adjusted by thesecond synchronization signal (W_(SN)).
 5. The method as claimed in oneof the preceding claims, wherein the xDSL transceivers (1-1; 1-2), forgenerating the transmit signals (W_(ECL), W_(SL), W_(SN), W_(OKN),W_(OKL)) are in each case supplied with a corresponding digitalsignaling data sequence which is scrambled by a scrambler (13) of thexDSL transceiver, mapped by a mapping unit (15) of the xDSL transceiver,coded by a coder (17) of the xDSL transceiver, converted into an analogsignal by a digital/analog converter (19) of the xDSL transceiver andamplified by a driver circuit (23) of the xDSL transceiver to thetransmit signal which is transmitted via a data transmission line (26).6. The method as claimed in one of the preceding claims, wherein ananalog signal (W_(ECL), W_(SL), W_(SN), W_(OKN), W_(OKL)), received froman xDSL transceiver by a data transmission line in each case amplifiedby a driver circuit of the xDSL transceiver, is converted into areceived digital signal by a digital/analog converter (28) of the xDSLtransceiver, has its echoes canceled by an echo cancelation circuit (21)of the xDSL transceiver, is equalized by an equalizer (33) of the xDSLtransceiver, and descrambled by a descrambler (37) of the xDSLtransceiver for further data processing.
 7. The method as claimed in oneof the preceding claims, wherein the echo adjustment signal (W_(ECL))and the first synchronization signal (W_(SL)) are generated fromdifferent signaling data sequences.
 8. The method as claimed in one ofthe preceding claims, wherein an xDSL transceiver, after receiving awarm start request signal, is brought from a standby mode into a datatransmission mode of operation.
 9. The method as claimed in claim 8,wherein an xDSL transceiver brought into the data transmission mode ofoperation first sends a wake-up signal (W_(UN), W_(UL)) to the otherxDSL transceiver via the data transmission line in order to also bringthe other xDSL transceiver from the standby mode into a datatransmission mode of operation.
 10. The method as claimed in one of thepreceding claims, wherein the data transmission link is set up when bothxDSL transceivers (1-1), (1-2) have been brought into the datatransmission mode of operation.
 11. The method as claimed in one of thepreceding claims, wherein after the synchronization sequence, user datasignals are transmitted between the two xDSL transceivers (1-1), (1-2)via the data transmission line (26).
 12. The method as claimed in one ofthe preceding claims, wherein the signaling data are in each case codedwith a Tomlinson code.
 13. The method as claimed in one of the precedingclaims 1 to 11, wherein the two synchronization signals (W_(SL), W_(SN))and the two indicating signals (W_(OKN), W_(OKS)) form a synchronizationsequence which can be repeated.
 14. An xDSL transceiver comprising asignal input (2) for receiving digital user data, a signal generator (8)for generating signaling data, a controllable multiplexer (3) forswitching through the generated signaling data or the user data independence on a control signal, a scrambler (13) for scrambling the dataswitched through by the multiplexer, a mapping unit (15) for mapping thescrambled data, a coder (17) for coding the mapped data, a D/A converter(19) for converting the coded data into an analog transmit signal, ahybrid circuit (25) for connecting the xDSL transceiver (1) via a datatransmission line (26) to a remote xDSL transceiver, an analog/digitalconverter (28) for converting an analog received signal into a digitalreceived signal, a detection circuit (40) for detecting a receivedwakeup signal, a synchronization circuit (35) for synchronizing to thereceived digital signal, an echo cancelation circuit (21) for echosignal cancelation in the received digital signal, an equalizer (33) forequalizing the received digital signal, a descrambler (37) fordescrambling the equalized received digital signal, and with a controlunit (10), the control unit (10), after a received wake-up signal hasbeen detected by the detection circuit (40) or after a warm startrequest signal has been received, controlling the coder (17) in such amanner that an echo adjustment signal W_(ECL) is sent by the xDSLtransceiver (1) via the data transmission line (26), the control unit(10), after receiving a first indicating signal (W_(ECOK)) from the echocancelation circuit (21), which indicates that the echo cancelationcircuit (21) is set, driving the multiplexer (3) in such a manner that agenerated synchronization data sequence is switched through to thescrambler (13) which is sent as synchronization signal (W_(SL)) by thexDSL transceiver (1) via the data transmission line (26) forsynchronizing the remote xDSL transceiver, the control unit (10), afterreceiving a second indicating signal (W_(SYNCOK)) from thesynchronization circuit (35), which indicates that the synchronizationcircuit (35) is synchronized to a received digital signal and afterreceiving a third indicating signal (W_(EQOK)) from the equalizer (33),which indicates that the equalizer (33) (EQ) is set, driving themultiplexer (3) in such a manner that a generated synchronization datasequence is switched through to the scrambler (13) which is sent asindicating signal (W_(OK)) by the xDSL transceiver (1) via the datatransmission line (26) to the remote xDSL transceiver and indicates tothe latter that the xDSL transceiver (1) is ready for the datatransmission of user data.